1. Field of the Invention
This invention relates to improved bearing materials comprising PTFE. These bearing materials are suitable for a variety of applications in, for example, the aerospace, industrial, medical and agricultural industries.
2. Description of Prior Art
It is known in the art to utilize self-lubricating bearings and materials to provide reduced friction and reduced wear in a range of load-bearing applications. These bearings are expected to withstand damage during use and installation. Further, the self-lubricating bearings are typically subject during use to a variety of conditions such as heat and pressure, as well as chemical attack from a variety of substances.
The choice of a bearing material to meet a given need depends on the specific conditions and performance required and tends to be a complex engineering task in view of the many parameters which must be taken into account. A representative list of conditions that are to be taken into account might include, for example, velocity, pressure (including amount of load, direction of load, and speed of impact of load), dynamic friction, static friction, temperature, chemical exposure, lubrication, dimensional stability, geometrical fit, nature of the counter surface, and susceptibility to fluid lubrication erosion (“cavitation”).
Conventional friction management materials and systems include roller bearings, ball bearings, and plain bearings. In the plain bearing arena, many different forms of plastics bearing materials comprising a plastic matrix having various fillers and/or porous bonding layers are known. Many of them include polytetrafluoroethylene (PTFE), which is widely known for its low coefficient of friction. PTFE also provides the benefit of being stable under a wide range of temperatures and is inert to most chemicals. However, the wear characteristics, excessive creep and the bond strength to substrates of PTFE are poor, so different supporting materials are incorporated with the PTFE in various ways. Some of these supporting materials include metals, which are believed to draw heat away from the system and thus result in improved wear. In addition, some metals, such as lead, are thought to contribute to the lubricity of the system. However, the science of mechanisms in these systems is not fully understood.
Many products have been made available in this field, and a variety of patents exist, directed to bearing and other friction-reducing materials incorporating polytetrafluoroethylene (PTFE). For example, many bearing materials incorporate PTFE floc, or short fibers, which are incorporated into a resin material and spray coated onto a substrate. U.S. Pat. No. 3,806,216 describes materials which are representative of this type of construction. In another form, PTFE film has been skived from a solid, full-density PTFE block, then laminated to fabric or metal backers and bonded together with various resin systems. U.S. Pat. No. 4,238,137, to Furchak, describes materials which are representative of this type of construction. PTFE fibers formed into woven or non-woven sheets or fabrics, which are then impregnated with resin (e.g., U.S. Pat. No. 4,074,512) and/or laminated to an epoxy or other backing material (e.g., U.S. Pat. No. 3,950,599) have also been used as bearing materials. PTFE floc or particles have been incorporated into a thermoplastic material, then molded and/or machined into bearings. Further, PTFE dispersions, sometimes combined with fillers, have been dried or otherwise bonded on a sintered metal layer/metal substrate or other metal substrate (e.g., U.S. Pat. Nos. 2,689,380; 5,498,654 and 6,548,188 and Japanese Unexamined (Kokai) Patent Application No. 3-121135).
U.S. Pat. No. 5,792,525 to Fuhr et al., teaches bearing parts formed from one or more layers of a densified expanded PTFE material which can be machined or otherwise formed to the desired shape. Such materials exhibit good resistance to creep under a load; however, the wear limitations of such materials limit their use in many demanding bearing applications.
As can be seen from the wide range of PTFE-containing materials described, some solution has been developed for virtually every bearing application; however, the market continues to need lower friction, lower wear systems that enable lower power consumption and longer bearing life. In addition, environmental concerns regarding lead have resulted in a search for lead-free materials that perform as well as, or better than, the current lead-containing materials.
Accordingly, a need has existed in the field of self-lubricated bearing materials and bearing articles for new bearings exhibiting enhanced wear resistance and low friction relative to conventionally available materials.